U.S. patent application number 13/737787 was filed with the patent office on 2013-07-18 for screw rotor for a screw type vacuum pump.
This patent application is currently assigned to VACUUBRAND GMBH + CO KG. The applicant listed for this patent is Vacuubrand GmbH + CO KG. Invention is credited to Jurgen Dirscherl, Frank Gitmans, Markus Prasse, Gerhard Ruster.
Application Number | 20130183185 13/737787 |
Document ID | / |
Family ID | 45531165 |
Filed Date | 2013-07-18 |
United States Patent
Application |
20130183185 |
Kind Code |
A1 |
Dirscherl; Jurgen ; et
al. |
July 18, 2013 |
SCREW ROTOR FOR A SCREW TYPE VACUUM PUMP
Abstract
A screw rotor is for a screw type vacuum pump, preferably for a
screw type vacuum pump having a pumping capacity less than 50
m.sup.3/h. The rotor has a rotor shaft, a rotor core which rests on
the rotor shaft, and a rotor cover which rests on the rotor core
and at least partially encloses the rotor core. The rotor core is
made of a material having a thermal conductivity greater than 100
W/mK, preferably a thermal conductivity greater than 200 W/mK. A
screw type vacuum pump has correspondingly designed rotors.
Inventors: |
Dirscherl; Jurgen;
(Kreuzwertheim, DE) ; Gitmans; Frank; (Wedemark,
DE) ; Ruster; Gerhard; (Hasloch, DE) ; Prasse;
Markus; (Stadtprozelten, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Vacuubrand GmbH + CO KG; |
Wertheim |
|
DE |
|
|
Assignee: |
VACUUBRAND GMBH + CO KG
Wertheim
DE
|
Family ID: |
45531165 |
Appl. No.: |
13/737787 |
Filed: |
January 9, 2013 |
Current U.S.
Class: |
418/201.1 ;
416/223R |
Current CPC
Class: |
F04C 29/0085 20130101;
F04C 2/16 20130101; F04C 29/0078 20130101; F04C 18/16 20130101;
F04C 29/04 20130101; F04C 25/02 20130101 |
Class at
Publication: |
418/201.1 ;
416/223.R |
International
Class: |
F04C 2/16 20060101
F04C002/16 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 12, 2012 |
EP |
12 000 151.6 |
Claims
1. A screw rotor for a screw type vacuum pump, the screw rotor
comprising: a rotor shaft, a rotor core which rests on the rotor
shaft, and a rotor cover which rests on the rotor core and at least
partially encloses the rotor core, wherein the rotor core is made
of a material having a thermal conductivity greater than 100
W/mK.
2. The rotor according to claim 1, wherein the rotor shaft is made
of a material having a thermal conductivity greater than 100
W/mK.
3. The rotor according to claim 2, wherein the rotor shaft and the
rotor core are designed as one piece.
4. The rotor according to claim 1, wherein the rotor core extends
into the screw threads of the rotor.
5. The rotor according to claim 4, wherein the rotor core extends
into the screw threads of the rotor only in an area of the rotor
which during operation faces an outlet of a pump chamber.
6. The rotor according to claim 1, wherein the rotor cover is made
of a material which has a low thermal conductivity compared to the
thermal conductivity of the rotor core and of the rotor shaft.
7. The rotor according to claim 6, wherein the rotor cover is made
of plastic.
8. The rotor according to claim 7, wherein at least one of the
rotor core, parts thereof, and the rotor shaft is made of copper,
aluminum, or alloys of these materials.
9. The rotor according to claim 1, wherein the rotor is configured
for a one-sided bearing at only one end of the rotor shaft.
10. The rotor according to claim 9, wherein in an area of the end
of the rotor facing away from the one end used for the bearing, the
rotor shaft has a reduced cross section, a recess, or is missing
completely, and a volume that is missing compared to an otherwise
complete outer dimension of the rotor, is filled by the rotor
cover.
11. The rotor according to claim 1, wherein a heat transfer means
for delivering heat to the ambient atmosphere is situated on the
rotor shaft at a distance from the rotor core.
12. A rotor according to claim 1, wherein the rotor core is made of
a material having a thermal conductivity greater than 200
W/m.times.K.
13. A rotor according to claim 4, wherein the rotor core, at a
location where the rotor core extends into the screw threads of the
rotor, has a reduced thickness.
14. The rotor according to claim 13, wherein the rotor cover has a
thickness of 0.1 mm to 10 mm in the location where the rotor core
extends into the screw threads of the rotor.
15. The rotor according to claim 6, wherein the rotor cover is made
of a material which has a thermal conductivity less than 5 W/m.
16. The rotor according to claim 7, wherein the plastic is
thermoplastic.
17. The rotor according to claim 7, wherein the plastic is a
chemically resistant plastic selected from the group consisting of
PPS, PEEK, and fluoroplastic.
18. The rotor according to claim 7, wherein the plastic is
reinforced with a filler selected from the group consisting of
carbon fibers and glass fibers.
19. The rotor according to claim 1, wherein the screw type vacuum
pump has a pumping capacity less than 50 m.sup.3/h.
20. A screw type vacuum pump comprising: a screw pump stator with
at least one inlet and one outlet, and two helical rotors which
rotate in mutual contactless engagement with one another in a
fittingly shaped pump chamber of the screw pump stator, and thus
convey a gaseous medium from the inlet to the outlet, wherein the
rotors each comprises: a rotor shaft, a rotor core which rests on
the rotor shaft, and a rotor cover which rests on the rotor core
and at least partially encloses the rotor core, wherein the rotor
core is made of a material having a thermal conductivity greater
than 100 W/mK.
21. The screw type vacuum pump according to claim 20, wherein the
outlet of the pump chamber is situated at an end of the pump
chamber facing the supported ends of the rotors.
22. The screw type vacuum pump according to claim 20, comprising a
dual-shaft synchronous drive for driving the rotors.
23. The screw type vacuum pump according to claim 20, wherein the
screw type vacuum pump has a pumping capacity less than 50
m.sup.3/h.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority of European Patent
Application No. 12 000 151.6, filed Jul. 12, 2012, which
application is incorporated herein by reference.
FIELD
[0002] The present disclosure relates to screw rotors for a screw
type vacuum pumps and screw type vacuum pumps.
BACKGROUND
[0003] Numerous processes in research and industry require a vacuum
in the range of 10.sup.2 Pa to 10.sup.-2 Pa (fine vacuum range),
wherein, frequently, condensing and/or aggressive vapors or gases
must also be conveyed. To generate a negative pressure in this
range, liquid-sealed or -lubricated vacuum pumps, such as
oil-sealed rotary vane pumps, are used. The use of such pumps, in
which the pumped medium comes into contact with oil or other
liquids, has many disadvantages. The pumped media may contaminate
the lubricant or react with it, which reduces the lubricating and
sealing effect. Backflow of gaseous components or decomposition
products of the lubricant into the process unit may significantly
interfere with the processes therein.
[0004] For this reason, for quite some time there have been efforts
to develop so-called "dry" vacuum pumps, i.e., pumps in which the
pumped media do not come into contact with a liquid.
[0005] At higher pressures, i.e., in the range of 10.sup.5 Pa to
10.sup.2 Pa, diaphragm vacuum pumps are advantageous, since the
pump chamber is hermetically separated from the drive area by the
diaphragm, which is clamped in a gas-tight manner. However, due to
the limited compression ratio and the valves which are usually
activated only by the gas flow, pressures below 50 Pa are difficult
to achieve.
[0006] In addition to fine vacuum pumps such as reciprocating
pumps, scroll pumps, claw pumps, and Roots pumps, screw type vacuum
pumps are known.
[0007] In screw type vacuum pumps (screw pumps for short), two
helical rotors intermesh with one another in a contactless manner
in a suitably shaped screw pump stator, so that due to their
counterdirectional rotation, gas is conveyed from an inlet to an
outlet. All statements in this regard, including in the discussion
below, relate to oil-free screw pumps having contactless
compression.
[0008] One advantage of the screw pumps is the high potential
compression, since screw pumps may intrinsically have a multistage
design in which each screw thread acts as a stage. Thus, screw
pumps provide the possibility of achieving a good ultimate vacuum
in the range of <1 Pa, using only one pair of rotors.
[0009] A so-called cantilevered bearing of this rotor pair is
possible in screw pumps. In a cantilevered bearing, the bearing is
provided from only one side of the rotor pair. No bearing is
present on the other side of the rotor pair. Thus, the screw pump
stator may be designed without a bearing unit. This allows simple
disassembly of the screw pump stator for maintenance and cleaning,
for example.
[0010] A general problem with screw pumps is the high heat release,
in particular in the area of the compression on the atmosphere
side. At low intake pressures, only a small quantity of gas is
conveyed from the suction side to the atmosphere side. Thus, there
is very little gas exchange inside the pump. In addition, a
negative pressure prevails up to the last atmosphere-side screw
thread in the pump chambers, which are formed by the intermeshing
screw threads.
[0011] When the pump chamber is open at the last screw thread on
the atmosphere side in the course of rotation of the rotor, gas
flows from the outlet back into this pump chamber. The inflowing
gas together with the gas that is conveyed here from the suction
port is expelled in the course of rotation of the rotors. This
pulsing of the gas at the outlet results in a high drive power
requirement, and releases large quantities of heat in a relatively
small volume.
[0012] The backflow of the gas may be reduced by end plates
situated tightly against the screw rotor and having openings at
appropriate positions. However, since these end plates at the same
time hinder the gas discharge, little improvement is gained by this
configuration.
[0013] Another approach for reducing the backflow is to provide
check valves at such end plates. However, these check valves must
open and close at the rotational frequency of the rotors. However,
the frequency of typically 6000-25,000 min.sup.-1 is usually too
high for this purpose; i.e., check valves having a sufficient size
respond too slowly.
[0014] To reduce the temperature and power problem, screw rotors
having a pump chamber volume which decreases toward the outlet are
commonly used. This may be achieved, for example, by a reduced
screw pitch or a reduced screw radius toward the outlet side. This
results in an internal compression of typically 2 to 10. The power
requirements of the pump and the heat release at the end of the
screw on the atmosphere side, based on the pumping capacity of the
pump, may thus be reduced almost by this compression factor.
[0015] A disadvantage of this method is that the manufacture of the
rotors becomes much more difficult due to the continuous or also
discontinuous change in the screw profile. Another disadvantage is
that the internal compression at high intake pressures may result
in internal overpressure. This may overload the drive motor and
cause damage to the pump. Complicated pressure relief valves in the
pump chamber stator in the area of the internal compression are
therefore often necessary. When noncompressible liquids are
conveyed, whether they are drawn in by suction or formed by
condensation in the interior, hydrostatic blockages may develop,
with the result that the pump abruptly stops due to overload. This
may result in costly consequential damage to the unit and the
drive.
[0016] Another approach is to use two separate screw pumps, having
different pumping capacities, connected in series, each on its own
having no internal compression (see EP 0 811 766 B1), whereby a
pressure relief valve may be connected between the pumps (see WO
2007/088989 A1). However, these approaches as well result in a high
level of design complexity (two pump units).
[0017] In known screw pumps of fairly large size, liquid cooling of
the pump housing is often used to control the thermal conditions.
For larger pumps, internal liquid cooling of the rotors is also
used, but this involves a high expenditure of effort.
[0018] In addition, it is not uncommon for gas from the outside to
also be admitted into the pump chamber in the area of the last
screw thread on the atmosphere side. The purge gas cools this area
and transports heated gas away from the last screw threads. The
high level of complexity as well as the unavoidable deterioration
of the ultimate vacuum of the pump are disadvantageous.
[0019] For compact screw pumps having typical rotor spacings of 20
to 100 mm and a pumping capacity <50 m.sup.3/h, internal liquid
cooling of the rotors cannot be used due to space and cost reasons.
In addition, liquid cooling of the housing would be disadvantageous
in such equipment, which should have flexible use in research
laboratories, for example, while the customary pumps, which are
much larger, for weight reasons are usually stationarily installed
in industrial facilities. Compact screw type vacuum pumps thus
require novel approaches to control the difficult thermal situation
at the end of the rotors on the atmosphere side.
[0020] Another aspect for compact screw type vacuum pumps is the
selection of the material of the rotors. Such screw rotors, which
are often designed in one piece with the rotor shafts, are usually
made of cast iron alloys or steel alloys, since these materials
have high rigidity (modulus of elasticity) and good machinability.
The thermal conductivity of this material class is only moderate,
but is generally adequate in conjunction with external water
cooling and optional internal oil cooling. In addition, rotor
temperatures of >150.degree. C. at the surface are still
acceptable for such materials.
[0021] A disadvantage of conventional steels and also cast iron
alloys is the limited chemical resistance. Aggressive chemicals
must be kept away from such pumps by means of cold traps or the
like. Furthermore, complicated operations using purge gas are
frequently involved. Nevertheless, when aggressive media are
conveyed, such pumps often have only short service lives.
[0022] Steel alloys having high chemical resistance, such as
Hastelloy, are usually difficult to machine, so that production of
the screw profiles, which often have complicated shapes and narrow
tolerances, is complex and expensive.
[0023] Another disadvantage of steel or cast rotors is their high
weight, which has an adverse effect on the required drive power
during acceleration, as well as the imbalance of the rotors.
Approaches for avoiding this problem with the aid of a rotor made
of aluminum on a steel shaft are known (DE 100 39 006 A1).
[0024] For applications using chemically aggressive substances,
rotors made of chemically resistant plastics would be advantageous.
Due to the limited rigidity (low modulus of elasticity) of
plastics, a shaft made of a more rigid material is generally
necessary inside the rotor. Such a system, composed of a rotor made
of plastic with a steel rotor shaft in the interior, is known (WO
2010/061939 A1).
[0025] A disadvantage of the latter-mentioned system is that
practically all usable plastics have a low thermal conductivity.
Even with a high filler content of carbon fiber, for example, it is
difficult to achieve a thermal conductivity greater than 1 W/mK.
For use in the screw pump, this means that with a high rate of heat
release at the end of the rotor on the atmosphere side, the heat is
not adequately dissipated, and the plastic material at that
location quickly heats to high temperatures. This may result in
excessive thermal expansion or even thermal damage (decomposition,
melting) of the material. The high rate of thermal expansion is
disadvantageous, since the rotors, which run by one another rapidly
(typically >6000 min.sup.-1) at a small clearance (typically
<0.1 mm), may then contact one another, which may result in
significant consequential damage.
[0026] The above-described problems with screw rotors for a screw
type vacuum pump have previously been addressed in the prior art
(GB 2 243 189 A). In the cited document, for an application in
conjunction with chemically aggressive substances in a screw type
vacuum pump, two rotors which run in mutual engagement are provided
which are made of cast iron, but provided with a thin coating
composed of protective materials, in particular plastic. The
problem of the high heat release and the dissipation of the heat is
not addressed therein. In fact, in the cited document, due to the
design of the rotor cores of the rotors made of cast iron, the
thermal conductivity is not high enough to actually protect the
plastic material from destructive heating. Since in this case the
rotor shaft is separate from the rotor core, i.e., the rotor core
is wedged onto the rotor shaft, here as well no design is disclosed
which represents an optimum solution for the heat dissipation.
SUMMARY
[0027] On the basis of the latter-mentioned prior art, the teaching
of the present invention is based on an object of providing a screw
rotor for a screw type vacuum pump in which use in the laboratory
under chemically aggressive conditions is in any case possible from
a design standpoint, and the above-described thermal problems are
likewise solved.
[0028] With reference to a screw type vacuum pump as a whole,
having two helical rotors in mutual contactless engagement with one
another in a pump chamber of a screw pump stator which is shaped to
fit, the above-described object can be achieved by the use of
appropriately configured rotors.
[0029] In summary, the following advantages result for the rotors
for compact screw pumps for use in research and industry, in
particular using chemically aggressive substances: [0030] Effective
cooling of the rotor is possible. [0031] The rotor has low thermal
expansion. [0032] The rotor shaft has sufficiently high rigidity
(modulus of elasticity). [0033] When a suitable material is used
for the rotor cover, the surface of the rotor may have high
chemical resistance and contact tolerance, i.e., without a tendency
toward scoring upon contact with the opposite screw rotor. [0034]
The rotor may be quite light in weight in order to reduce potential
imbalances. [0035] When suitable materials are used for the rotor
cover, the production of the screw profile, which is often very
demanding and associated with narrow tolerances, is simplified by
good machinability of the rotor material.
[0036] The present inventors have realized that the cooling of the
screw rotor for screw pumps having a compact design must be carried
out primarily by discharging heat from the pump chamber via the
rotor and the rotor shaft. For this purpose it is provided to
construct the rotor with a rotor core made of a highly thermally
conductive material which is surrounded by a rotor cover,
preferably made of a chemically resistant plastic. Materials having
a thermal conductivity greater than 100 W/mK, preferably greater
than 200 W/mK, for example aluminum or copper and some alloys, are
used as highly thermally conductive material. Plastics and iron
alloys (steel, cast iron) do not achieve these values, and do not
have sufficient heat dissipation for effective cooling of the rotor
toward the interior or via the rotor shaft.
[0037] In one design, the rotor shaft is also made of a highly
thermally conductive material, so that the heat is transported from
the pump chamber by solid-state heat conduction from the rotor via
the rotor shaft. It is particularly advantageous if the rotor shaft
is designed in one piece with the rotor core, since in that case
the solid-state heat conduction occurs inside the rotor core,
without interfering interfaces, into the rotor shaft and to the
outside.
[0038] In an alternative design, instead of a solid, highly
thermally conductive material a hollow shaft is used for the rotor
shaft, through which a cooling gas such as air is conveyed, and
which is drawn in by the rotation of the shaft itself, for example
by a type of blower on a free end of the shaft. The cooling gas is
led through the rotor shaft to the area of the highest heat
release, and at that location cools the highly thermally conductive
rotor core from the inside. The heated gas is delivered into the
outlet of the pump, for example, where it may be used as a purge
gas, or is recirculated back through the rotor shaft. Thus, in this
design as well, the rotor is made of a highly thermally conductive
rotor core which is surrounded by a rotor cover, the rotor core
being in contact with the hollow rotor shaft or in direct contact
with the cooling gas. The material of the hollow rotor shaft may
also be highly thermally conductive.
[0039] The thickness of the casing material, i.e., the rotor cover,
results from the need, on the one hand, for the layer to be
diffusion-proof and mechanically stable, and on the other hand, for
the thermal conduction through the layer to the core material to
still be great enough to avoid overheating at the surface. This
means that in one design the highly thermally conductive core
material extends into the screw threads, and is not present only as
an essentially cylindrical part. This means that the highly
thermally conductive material, at least in this section, has the
screw profile (reduced by the casing wall thickness), the casing
material in these areas preferably having a thickness of 0.1 to 10
mm.
[0040] Although the rotor cover often has only a comparatively low
specific thermal conductivity of usually <5 W/mK (typical for
plastics, for example), sufficient heat dissipation through this
layer to the rotor core is achieved on account of the small
thickness of the rotor cover. As a result of the application, the
highly thermally conductive rotor core extends outwardly into the
screw threads preferably in the area of the highest heat release,
i.e., at the end of the rotor on the atmosphere side, in which a
high level of heat dissipation through the rotor is necessary.
[0041] However, in one embodiment the rotor has one or more
sections in which the highly thermally conductive rotor core does
not extend, or does not extend completely, outwardly into the screw
threads. Possible materials for the rotor core and the rotor cover
are considered for purposes of explanation.
[0042] The rotor core may be encased in various ways. If a very
thin coating (<0.1 mm) is applied, under some circumstances
mechanical refinishing of the layer may be dispensed with. However,
such layers are often not completely diffusion-proof, so that the
layer may be infiltrated by the pumped media and then chip off
under vacuum. For thicker coatings, the shape of the screw profile
must be laboriously refinished. Thicker coatings are usually fused
on after the application (for example, by electrostatic powder
coating). This often leads to rounding of the edges, resulting in
defects at the outer edges after the final machining.
[0043] A distinction is made between these coating processes and
the extrusion coating of a rotor core with a thermoplastics
plastic. In this process, the layer thickness may be selected
practically as desired (i.e., so that it is also diffusion-proof),
and the edges are precisely formed. At the same time, this process
also allows filling of fairly large plastic volumes.
[0044] A comparison of the mechanical and thermal parameters of
various materials shows that of the materials having a very high
thermal conductivity of >100 W/mK, copper and some copper alloys
appear to be very suitable. The reason is the high thermal
conductivity, the still acceptable thermal expansion, and the still
acceptable modulus of elasticity. Aluminum and its alloys show much
less favorable values for all three parameters, but are lighter in
weight. Due to the much lower modulus of elasticity, aluminum is
not very suitable as a rotor shaft material, but may be used as a
rotor core material, in which case the rotor shaft would have to be
made of a different material such as copper, or be composed of a
hollow shaft having internal gas cooling. For protection of
corrosion-sensitive materials such as copper, a coating with Ni,
Cr, Ag, or Au, for example, may be provided.
[0045] Other metals having high thermal conductivity, such as gold,
silver, alkali metals and alkaline earth metals, zinc, molybdenum,
or tungsten and their alloys are ruled out because of excessive
material costs, poor machinability, reactivity, or low modulus of
elasticity. Novel materials such as CFRP often have anisotropic
properties which are difficult to control, in particular in the
shaping of solid bodies. Furthermore, the manufacture is often
expensive and complicated. In addition, specialized ceramics such
as AlN have interesting material properties, but are difficult to
machine. Nevertheless, these materials are of interest in the
future for the rotor core or parts of the rotor core of rotors
according to the invention.
[0046] Disadvantages of copper are its high specific weight and
comparatively high material cost. Therefore, the second aspect of
the invention, namely, that the highly thermally conductive core
material extends outwardly to just under the plastic surface only
where this is thermally necessary, i.e., in particular in the area
of the compression on the atmosphere side, is particularly
advantageous. In other areas of the rotor which are subject to less
thermal stress, the formation of the core material until it reaches
into the screw threads may be dispensed with, and at such location
the rotor may be composed of a relatively small cylindrical rotor
core that is surrounded by plastic as the rotor cover.
[0047] Chemically highly resistant plastics such as PPS, PEEK, or
fluoroplastics, which preferably are reinforced with fillers such
as carbon fiber or glass fiber, are preferably used as casing
material. For example, the density of PEEK with carbon fiber
reinforcement is only approximately 16% that of copper. Thus, by
means of the system in which the highly thermally conductive core
material extends outwardly to just under the plastic surface only
where this is thermally necessary, i.e., in particular in the area
of the compression on the atmosphere side, the weight of the screw
rotor and thus, potential imbalances, may be significantly reduced.
Profile-related imbalances may be largely compensated for at the
rotor core itself, so that only minor corrections to the rotor
cover are necessary at the completed rotor, and large balancing
rings or boreholes may be dispensed with.
[0048] Further advantages of materials such as PPS, PEEK, or
fluoroplastics are the good machinability and the contact
tolerance, i.e., a low tendency toward scoring. The machining of
such plastics is much simpler and quicker, and thus more
cost-effective, than highly corrosion-resistant stainless steels,
for example.
[0049] The system thus results in a rotor having a chemically
highly resistant and diffusion-proof surface, and at the same time,
very high thermal conductivity of the overall system, at least in
the area of high heat release during operation, with surprisingly
favorable manufacturing costs. The latter results due to the fact
that materials such as copper or aluminum are used only where
necessary, material-conserving manufacturing processes such as
injection molding are used, and the materials are easily
machinable.
[0050] For reliably connecting the rotor cover to the rotor core, a
positive-fit connection, preferably having undercuts for
interlocking, is necessary if adequate material adhesion is not
achievable. For this purpose, grooves, holes, or channels, for
example, may be introduced into the rotor core. A rough surface of
the rotor core, achieved by sandblasting, for example, is also
helpful.
[0051] The advantages of the rotor are particularly apparent in the
preferred configuration of the screw pump having cantilevered
rotors. For cantilevered rotors, the bearing and drive area is
preferably under ambient air pressure, and is not in contact with
the pumped media. To keep this bearing and drive area from having
to be sealed off from the pump chamber by a shaft sealing ring or
the like, the pressure side of the pump unit is usually situated on
the drive side.
[0052] This area is doubly subjected to thermal stress: on the one
hand from the motor, and on the other hand from the heat of
compression at the end of the screw rotor on the atmosphere side.
However, when high-efficiency synchronous motors or a gearing
is/are used, and when the drive area is efficiently cooled using a
blower, for example, the drive area may be kept at low operating
temperatures fairly easily.
[0053] The waste heat from the compression may be several times
that of the motor waste heat. The rotor design now allows very
effective dissipation of the heat of compression from the pump
chamber in the direction of the well-cooled drive area, with the
aid of the highly thermally conductive rotor shaft made of a solid
material.
[0054] In one design, a means for delivering this heat to the
ambient air is situated in the drive area, on the rotor shaft. This
may be, for example, a co-rotating fan impeller or disks made of
copper or aluminum, for example. These elements deliver the heat
from the rotor shaft to the air very effectively due to the rapid
rotation. The heated air may be discharged via an externally
applied cooling air flow. An air flow generated by a co-rotating
fan impeller may also be used for cooling the motor.
[0055] In particular for cantilevered rotors, it is important that
the lowest possible weight is present at the end of the rotors
remote from the bearings. Motions of large masses at a large
distance from the bearing, even with slight imbalance, may result
in great deflections, and thus, rotor collisions.
[0056] In this case a second aspect becomes particularly important,
according to which the highly thermally conductive rotor core
(often having a high density) outside the highly thermally stressed
part of the rotor, is not drawn to just under the plastic surface,
but, rather, is reduced to the greatest extent possible. This
significantly reduces the moved mass specifically at the end of the
rotor remote from the bearing.
[0057] For the dimensionally accurate machining of the cantilevered
rotors (in the pump), depending on the production method it is also
necessary to hold the rotor in the machining tool on the side
facing away from the bearing. If the rotor cover is not suited for
this purpose, the highly thermally conductive rotor core may also
be guided outwardly on the end-face side, on the side facing away
from the bearing. If necessary, this area must subsequently be
protected from corrosion attack, for example by covering with a
plug made of PTFE, for example. Alternatively, the holding at the
end face may also be carried out using a highly corrosion-resistant
metal such as Hastelloy which is fixedly joined to the core
material.
[0058] In an alternative form of the rotor for a cantilevered
bearing, the highly thermally conductive rotor core and/or the
rotor shaft is/are not present with the complete cross section over
the entire length of the screw rotor, or is/are hollow or absent
altogether. The part of the rotor facing away from the bearing may
then be made of solid casing material or have a recess. All of
these characteristics result in a marked reduction of the moved
masses in the area of the rotor remote from the bearing.
[0059] The highly thermally conductive rotor core, for example made
of copper or aluminum or an alloy thereof, may be manufactured from
the solid material, or preferably by affixing a hollow screw to a
shaft or by joining a solid screw having a short shank, both of
which reduce the material expenditure for the manufacture. In one
design with even less material usage, the rotor core as a whole or
the hollow screw is pre-cast, or the latter is made of an
appropriately curved sheet metal part.
[0060] In another embodiment, additional functional elements of the
rotor are integrated into the rotor cover. These may be, for
example, balancing weights on one or both sides of the screw, or
also purge gas fans as disclosed in DE 10 2010 055 798 A1.
[0061] In another embodiment, the drive of the screw pump is
achieved by a dual-shaft synchronous drive composed of magnetized
cylinders on each of the two rotor shafts, which due to their
mutual magnetic interaction synchronize the rotors in opposite
directions. The two magnetized cylinders are enclosed by one or
more windings, which when suitably energized generate migrating
magnetic fields so that the two magnetized cylinders, and thus the
rotor shafts, rotate synchronously in opposite directions.
[0062] The present disclosure is explained in greater detail below
with reference to drawings which illustrate exemplary embodiments
strictly by way of example. The drawings show the following:
[0063] FIG. 1 shows a screw rotor for a screw type vacuum pump, in
cross section; and
[0064] FIG. 2 shows a screw type vacuum pump having two rotors, in
cross section.
[0065] FIG. 1 shows a screw rotor 1 in cross section. The rotor 1
is intended for use in a screw type vacuum pump, for example in a
screw type vacuum pump having a pumping capacity less than 50
m.sup.3/h.
[0066] The rotor 1 schematically illustrated in cross section in
FIG. 1 basically consists of a rotor shaft 2, a rotor core 3 which
rests on the rotor shaft 2, and a rotor cover 4 which rests on the
rotor core 3. As illustrated, the rotor shaft 2 is separate from
the rotor core 3. In principle, it is also possible for the rotor
shaft 2 and the rotor core 3 to be designed as one piece.
[0067] The rotor cover 4 at least partially encloses the rotor core
3. In the exemplary embodiment illustrated in FIG. 1, the rotor
cover 4 encloses the rotor core 3 on the rotor shaft 2 at all outer
surfaces, i.e., at all surfaces which do not abut against the rotor
shaft 2.
[0068] In the illustrated exemplary embodiment, the rotor core 3 is
made of a material having a high thermal conductivity greater than
100 W/mK, preferably a thermal conductivity greater than 200 W/mK.
In addition, the rotor shaft 2 is preferably made of a material
having a high thermal conductivity, in the present case, preferably
a thermal conductivity greater than 100 W/mK.
[0069] Alternatively or additionally, the rotor shaft 2 may have
one or more channels extending parallel to the axis of the rotor
shaft for supplying gas in the direction of the rotor core 3, so
that the rotor 1 as a whole is cooled from the inside.
[0070] The rotor core 3 in individual sections of the rotor 1 may
extend into the screw threads thereof, as illustrated in region 5
in FIG. 1. Thus, the rotor core 3 then has practically the same
outer dimensions as the rotor 1 as a whole, with only a thin layer
which forms the rotor cover 4. In this area, thicknesses of the
rotor cover 4 between 0.1 mm and 10 mm are conceivable. This design
is implemented in particular at locations where significant heat
develops in a screw type vacuum pump during operation of the rotor
1, thus, in particular where the compression occurs at atmospheric
pressure, near the outlet of a pump chamber of a screw type vacuum
pump.
[0071] In less stressed areas, the rotor core 3 may be entirely
absent, so that the rotor cover 4 may form the complete rotor 1
outside the rotor shaft 2. This is apparent in region 7 at the top
in FIG. 1.
[0072] With regard to the rotor cover 4, it can be made of a
material which has a low thermal conductivity compared to the
thermal conductivity of the rotor core 3 and of the rotor shaft 2,
for example a thermal conductivity less than 5 W/mK. In particular,
it is recommended here that the rotor cover 4 is made of plastic,
in particular a thermoplastic plastic. In appropriate uses for
chemical applications, the selection of a chemically resistant
plastic such as PPS, PEEK, or fluoroplastic is recommended. The
strength of the plastic of the rotor cover 4 may be increased using
fillers such as glass fibers or carbon fibers.
[0073] The rotor cover 4 is joined to the rotor core 3, i.e.,
mounted thereon, in an injection molding process. Copper or
aluminum or alloys of these materials are recommended as materials
for the rotor core 3 or parts thereof, and/or for the rotor shaft
2.
[0074] FIG. 1 shows the rotor shaft 2 of the rotor 1 protruding at
both ends, i.e., projecting significantly with respect to the rotor
core 3 and the rotor cover 4. The rotor 1 is supported at both
ends.
[0075] In contrast, the rotors 1, 1', which are installed in the
screw type vacuum pump as illustrated in FIG. 2, are configured for
a one-sided bearing at one end. In this case, the rotor shaft 2 has
a significant projection with respect to the rotor core 3 and the
rotor cover 4 only at its end used as the bearing, namely,
protrudes into a bearing area.
[0076] In the rotors 1, 1' in FIG. 2, it is also apparent that in
the area of the end of the rotor 1, 1' facing away from the end
used for the bearing, the rotor shaft 2 and/or the rotor core 3,
depending on the distance from the end used for the bearing,
has/have a reduced cross section, a recess, or is/are missing
completely. The volume missing compared to the complete outer
dimensions of the rotor 1, 1' is filled by the rotor cover 4.
[0077] FIG. 2 shows a schematic sectional view of a screw type
vacuum pump having helical rotors 1, 1' in mutual contactless
engagement with one another, inserted therein. The screw type
vacuum pump in FIG. 2 has, first of all, a screw pump stator 8
which essentially forms the housing of the screw type vacuum pump.
The screw pump stator 8 contains a pump chamber 9, shaped to fit
the rotors 1, 1', which has at least one inlet 10 and one outlet
11. The gaseous medium is conveyed from the inlet 10 to the outlet
11 by the contactless rolling off of the two counter-rotating
rotors 1, 1' in the appropriately shaped pump chamber 9. The rotors
1, 1' together with the rotor shaft 2, rotor core 3, and rotor
cover 4 are configured in the same way as described in detail for
the rotor 1 illustrated in FIG. 1.
[0078] The rotors 1, 1' in the exemplary embodiment in FIG. 2
differ from the rotor 1 in FIG. 1 in that the rotors 1, 1' are
cantilevered, i.e., supported only on one side. There is no bearing
on the opposite end of the rotors 1, 1', i.e., at the top in FIG.
2.
[0079] In FIG. 2, a bearing and drive area in which the rotor
shafts 2 of the rotors 1, 1' are supported is situated beneath the
pump chamber 9 in the screw pump stator 8. It is apparent that the
outlet 11 of the pump chamber 9 is situated at the end of the pump
chamber 9 facing the supported ends of the rotors 1, 1'.
[0080] The bearing and drive area is preferably under ambient air
pressure. This area contains means 12, 12'; 13, 13' for bearing the
rotors 1, 1', and means for synchronizing and/or for driving the
rotors 1, 1'. The means 12, 12'; 13, 13' can be for example roller
bearings or ball bearings. FIG. 2 depicts roller bearings. In the
example illustrated here, the latter means are composed of
appropriately magnetized cylinders 14, 14' which due to their
mutual magnetic interaction synchronize the rotors 1, 1' in
opposite directions. The two magnetized cylinders 14, 14' are
surrounded by one or more windings 15, 15', which when suitably
energized generate migrating magnetic fields so that the two
magnetized cylinders 14, 14', and thus the rotor shafts 2 of the
rotors 1, 1', rotate synchronously in opposite directions. Thus, in
the present case the drive of the screw type vacuum pump is
configured as a dual-shaft synchronous drive 14, 14'; 15, 15'.
These types of designs are known per se from the prior art.
[0081] Illustrated on the shaft in the drive area are heat transfer
means 16, 16' for discharging heat, which has been conducted here
via the rotor shafts 2, to the ambient air. These may be
co-rotating fan impellers or disks, for example. The heated air may
be discharged via an externally applied cooling air flow (not
illustrated). The air flow generated by the co-rotating heat
transfer means 16, 16' may also be used for cooling the drive 14,
14'; 15, 15'.
[0082] In addition, further functional elements 17, 17' are
indicated in FIG. 2 which may be used for balancing, for example.
Functional elements 17, 17' attached to the rotor cover and can be
balancing weights which function to counter-balance the rotor
itself, fan wheels, and/or the like. In one example, these may be
purge gas fans for drawing in purge gas from the bearing area and
thus flushing the bearings.
[0083] In the end region 18 illustrated in FIG. 2, the rotor cover
4 of the respective rotor 1, 1' has axially inwardly extending
recesses. Beneath the recesses, the rotor cover 4 of both rotors 1,
1' in each case extends over the complete cross section of the
rotor 1, 1', transversely with respect to the axis of the rotor 1,
1', since the respective rotor shaft 2 terminates just below this
area.
* * * * *